1. Field of the Invention
The invention concerns a device for the sputter application of hard material coatings at high temperatures. A preferred area of application is the coating with hard material of forming tools and Cutting tools by high-rate sputtering using magnetrons. More particularly, the invention concerns devices for coating temperatures of 400.degree. C. to 800.degree. C.
2. Description of Background and Relevant Information
It is generally known to coat workpieces, especially tools for the cutting of metals, and forming tools with hard, wear-resistant, protective coatings by means of physical vapor deposition (PVD). The most frequently employed PVD processes for these applications are vacuum arc evaporation and magnetron sputtering. Coatings of titanium nitride continue to enjoy the most widespread use (Sue, J. A.; Troue, H. H.; Friction and wear properties of titanium nitride coating in sliding contact with AISI 01 steel; Surf. Coat. Technol. no. 43/44 (1990) p. 709-720).
In addition, a large number of other coatings, such as nitrides, carbides, carbonitrides, and oxynitrides of the metals chromium, niobium, zirconium, titanium, etc., arc known as hard material coatings (DE 295 10 545; U.S. Pat. No. 4,169,913).
Moreover, extensive efforts are underway to use diamond-like carbon layers to achieve hard and wear-resistant coatings on workpieces (EP 0 503 822).
All these coatings applied by means of PVD are deposited at comparatively low temperatures, i.e., in the range between room temperature and approximately 400.degree. C.
Known PVD devices, as they are used for coating by means of sputtering sources, have one or more magnetron sources for the coating material. The workpieces to be coated are held in fixtures, and rotate about one or more axes during the coating process. Adequate uniformity of the coating is ensured in this manner. The largest number of workpieces per coating process and the greatest uniformity of coating thickness are achieved when the workpieces are arranged on a rotating device in fixtures that are part of planetary drives and thus rotate about the primary axis of the planetary drive and also about their own axes. For economic reasons, the diameter of these fixtures is chosen to be as large as possible. Consequently, their diameter is approximately 30 to 40 percent of the diameter of the rotating device.
Arranged on the circumference of such PVD devices, outside the region of the rotating fixtures, are laminar heating devices. They customarily consist of heating elements that are embedded in stainless steel tubes and are electrically insulated therefrom. In order to ensure workpiece temperatures of 400.degree. C. during coating, surface temperatures of the heating devices of at least 600 to 700.degree. C. are necessary, and a noticeable portion, as a rule, 20 to 30 percent, of the circumference of the rotating device must be enclosed by the heating device. In this situation, it is unavoidable that a large portion of the heat output reaches the walls of the vacuum chamber. For this reason, the walls are equipped with water cooling. After the coating process is completed, the vacuum chamber is ventilated. The coated workpieces are removed from their fixtures and other workpieces to be coated are inserted. During this process, the rotating device can be disconnected from the drive elements and removed from the vacuum chamber as a unit. The heating devices always remain in the vacuum chamber. The shortest changeover times are achieved when a second rotating device filled with workpieces stands ready and is loaded into the vacuum chamber.
The disadvantages of such devices arc that the power density of the heating elements cannot be increased sufficiently for a workpiece temperature of 800.degree. C. to be reached. In other words, they cannot be used at temperatures above approximately 500.degree. C. Furthermore, even in the temperature ranges up to 500.degree. C., the temperature differences among the workpieces as well as within each individual workpiece increase with increasing temperature. At the same time, the temperature of the individual workpiece is subject to large variations over time. Thus, supercritical internal stresses arise in the deposited coatings, which lead to flaking of the coatings.
The fact that the fixtures, the rotating device, the heating devices, and all internal surfaces of the vacuum chamber are coated as well the workpieces, leads to the further major disadvantage that coating particles are released from these surfaces. These coating particles, known as flakes, some of which are electrically charged, lead to substantial problems since they adversely affect the quality of hard material coatings deposited by sputtering to such a degree that the service life of workpieces coated in this manner is very short.
Lastly, the heat output available to heat the workpieces is reduced by the layers precipitating on the heating devices, and the heat losses toward the walls of the vacuum chamber take on unacceptably high values. Cleaning of the device, especially of the heating devices, involves difficulties.